Friction stir welding in plastics?

I'm familiar with the process of friction stir welding, which has been used with great success for assembling the fuel tanks for satellite launchers like the Delta IV and the Atlas V. However, I was wondering if, from a theoretical aspect, it would be possible to friction stir weld together plastics. I know that it'd be tough, if not outright impossible, with fiber-reinforced plastics. It's also not practical, when you can simply mold the plastic into shape in the first place. But the question remains:

No reason why not, provided the plastic is not thermosetting.
However, that means stuff such as the matrix material for aerospace composites is not suitable.
Polyethylene plastics however would be ok, although there may be substantial material property changes if the friction stirring decrystallises the welding zone.